The term "electrography" and "electrographic" as used herein broadly include various processes that involve forming and developing electrostatic charge patterns on surfaces, with or without the use of light. They include electrophotography and other processes. One method of electrographic development is the magnetic brush method which is widely used for dry development in electrophotographic document copying machines. It is disclosed, for example, in U.S. Pat. No. 3,003,462. The method of the present invention is useful in preparing the carrier particles for two-component dry developers used in the magnetic brush method. Such a developer is a mixture of thermoplastic toner particles and magnetic carrier particles, the latter being partially coated with an insulating resin.
In the development station of a copying machine, the two-component developer, which includes the magnetic carrier particles, is attracted to a magnetic brush consisting of stationary magnets surrounded by a rotating cylindrical sleeve or a stationary sleeve surrounding rotating magnets, e.g., as in the patent to Miskinis et al., U.S. Pat. No. 4,546,060. By frictional contact with the resin-coated carrier particles the toner particles are triboelectrically charged and cling to the carrier particles, creating bristle-like formations of developer on the magnetic brush sleeve. In developing a charge pattern the brush is brought close to the charged surface. The oppositely charged toner particles are drawn away from the carrier particles on the magnetic brush by the more strongly charged electrostatic charge pattern, thus developing and making visible the charge pattern.
Although uncoated iron particles have been used as carriers in magnetic brush developers and although the high conductivity of uncoated iron particles is desirable because a conductive magnetic brush serves as a development electrode and improves the development of large solid areas in the image, nevertheless, resin-coated carrier particles have been preferred. One reason for resin-coating the carrier particles has been to improve the triboelectric charging of the toner particles. When a resin-coated carrier is used, the toner powder acquires an optimally high, net electrical charge because of the frictional contact of the toner particles and the resin coating. The high net charge reduces the amount of toner lost from the developer mix as it is agitated in the magnetic brush apparatus.
Especially useful as the carrier particles in two component developers are strontium and barium ferrites. However, a problem that has been encountered with magnetic ferrite carrier particles containing strontium and barium has been the contamination of the carrier particles with dust or fines in the form of strontium or barium oxides. When such a carrier is mixed with toner powder to form the two-component developer mixture, this dust deposits on the surfaces of the toner particles and reduces their ability to develop an electrostatic charge. An indication of such contamination is toner "throw-off" during the development process. Throw-off is the term used to describe toner particles that separate from the carrier before they are attracted to the more strongly charged photoconductor. This phenomena may also be described as "early life dusting."
In the past, a way of overcoming this problem has been to precondition the developer. This is done by agitating or "exercising" the developer mixture for a period of time before it is used for electrostatic image development and stripping off the contaminated toner particles. However, a problem can result from this technique in that the resinous toner particles tend to scum the carrier or pack into its pores. When this happens the capability of the carrier for triboelectric charging of the toner particles is adversely affected.